This invention relates generally to high power solid state amplifiers and more particularly to an improved microstrip implementation configured to enhance heat dissipation and optimize isolation between input and output circuitry.
Conventional microwave solid state amplifiers employ microstrip transmission lines to deliver drive power to microwave output transistors and to connect the transistors to a useful output port. Many applications employ a plurality of such amplifiers arranged in modular fashion to produce high power output levels. For example only, certain microwave signal transmission systems utilize over one hundred microstrip amplifier modules to produce in excess of 70 kilowatts of power. A suitable amplifier module can use a single input transistor to drive multiple output transistors via microstrip circuitry formed on a substrate supported in a sealed housing. Conventionally, the transistors and input-output microstrip circuitry are supported adjacent a common substrate surface which bounds a common cavity in the housing. Typically, the common cavity can support many wave guide modes which can cause undesired feedback resulting in amplifier instability. Moreover, such conventionally constructed circuitry is often characterized by relatively long thermal paths between the output transistors and a heat sink surface. This can result in undesirably high transistor junction temperatures which limit power output and device reliability.
The present invention is directed to a solid state amplifier implementation comprised of a conductive substrate defining first and second parallel planar surfaces and an edge surface extending substantially perpendicular to said planar surfaces. In accordance with the invention one or more output transistors is physically mounted adjacent to said edge surface to enable a heat sink to be conveniently placed in close proximity thereto to provide a short thermal path and enhanced heat dissipation.
In accordance with a preferred embodiment of the invention, input circuitry is located adjacent to said substrate first planar surface and output circuitry is located adjacent to said substrate second planar surface. The input and output circuitry is formed, at least in part, by microstrip, i.e., strip transmission, lines, supported on respective dielectric layers deposited on the substrate first and second planar surfaces. The substrate forms a common ground plane.
In accordance with a further aspect of the preferred embodiment, the substrate bearing the output transistor(s) and input I output circuitry is mounted in a conductive housing including a removable cover plate. With the cover plate and substrate in place, the first substrate surface carrying input circuitry is contained in a first cavity isolated from the second substrate surface carrying output circuitry and contained in a second cavity. This isolation enhances amplifier stability.